The present invention relates generally to wireless communication systems and adaptive methods of operating such systems in a multipath fading environment.
Wireless communication systems serving stationary and mobile wireless subscribers are rapidly gaining popularity. Numerous system layouts and communications protocols have been developed to provide coverage in such wireless communication systems.
In mobile systems, for example, a variety of factors cause signal degradation and corruption. These include interference from other cellular users within or near a given cell. Another source of signal degradation is multipath fading, in which the received amplitude and phase of a signal varies over time. The fading rate can reach as much as 200 Hz for a mobile user traveling at 60 mph at PCS frequencies of about 1.9 GHz. In such environments, the problem is to cleanly extract the signal of the user being tracked from the collection of received noise, CCI, and desired signal portions summed at the antennas of the receiver.
It is also known in the communication art that transmit units and receive units equipped with antenna arrays, rather than single antennas, can improve receiver performance. Antenna arrays can both reduce multipath fading of the desired signal and suppress interfering signals or CCI. Such arrays can consequently increase both the range and capacity of wireless systems. This is true for wireless cellular telephone and other mobile systems as well as Fixed Wireless Access (FWA) systems.
In FWA systems, e.g., where the receiver and transmitter remain stationary, signal fading rate is typically less than in mobile systems. Accordingly, in FWA systems the channel coherence time (time during which the channel state estimate remains stable) is increased, since the transmitter and receiver do not move. Nevertheless, over time channel coherence is typically lost in FWA systems as well.
Antenna arrays enable a system designer to increase the total received signal power, which makes extraction of a desired signal easier. Signal recovery techniques using adaptive antenna arrays are described in detail, e.g., in the handbook of Theodore S. Rappaport, Smart Antennas, Adaptive Arrays, Algorithms, and Wireless Position Location; and Paulraj, A. J et al., xe2x80x9cSpace-Time Processing for Wireless Communicationsxe2x80x9d, IEEE Signal Processing Magazine, November 1997, pp. 49-83.
U.S. Pat. No. 5,687,194 to Paneth et al. describes a proposed Time Division Multiple Access (TDMA) communication system using multiple antennas for diversity. U.S. Pat. No. 5,952,963 to Qun Shen et al. discloses a system for using antenna selection diversity. The antenna selection is based on the receive signal strength indicators (RSSI) measured during the transmission of a preamble.
xe2x80x9cSpace-time codes for high data rate wireless communication: Performance criterion and code construction,xe2x80x9d by V. Tarokh et al, IEEE Transactions on Information Theory, V.44, No. 2, March 1998, describes joint design of coding, modulation, transmit and receive diversity to provide high performance. xe2x80x9cA simple transmit diversity technique for wireless communications,xe2x80x9d by S. M. Almouti, JSAC V.16, No. 8, October 1998, describes a simple two branch transmit diversity scheme. In this scheme, at a given symbol period, two symbols are simultaneously transmitted from two antennas. During the next symbol period, each antenna transmits a conjugate of the symbol transmitted by the other antenna during the previous symbol period. This works almost like receive maximal ratio combining, but under the condition that the channel does not change from symbol period to symbol period. The above condition, however, is not always true.
In a multipath environment, the signal arrives at the receive antenna via multiple paths. These paths can be a combination of line of sight (LOS) and nonline of sight (NLOS) paths. When the composite received signal consists of a large number of plane waves, the received complex low pass signal can be modeled as a complex Gaussian random process. In the absence of a LOS signal component, the received signal amplitude has a Rayleigh distribution. This type of multipath fading is called Rayleigh fading. When the multipath fading channel has a LOS component, the signal amplitude has a Rician distribution. This is called Rician fading. The Rician factor K is the ratio of received power in the LOS component to that in the scattered NLOS components. When K=0, the channel exhibits Rayleigh fading, and when K=∞, the channel does not exhibit fading. Antenna diversity is a traditional technique to combat multipath fading, but any single diversity technique is not suitable for all channel conditions. For example, delay diversity using multiple antennas is suitable when there is considerable fading (low K-factor) and delay spread in the channel. But when the channel is Rician (with high K values), the delay diversity for example can create or aggravate intersymbol interference (ISI) and frequency selective fading of a permanent nature, i.e., delay diversity can cause certain carrier frequency components to have very low signal level.
One way to solve this problem is to not use diversity when the channel is Rician. But for this, the knowledge of the channel characteristics prior to transmission is required. When such feedback is not possible, communication will be very error prone. None of the above-referenced related art teaches or suggests a method or system to overcome this latter problem.
There is a need, therefore, for a method and a system to carry out robust wireless communication through a channel in a multipath fading environment under varying channel conditions ranging from pure Rayleigh to pure Rician and varying delay spread. Particularly, there is a need to provide such a method and system to carry out robust wireless communication under conditions wherein a transmitter has no channel state information. Further, there is a need to provide a method and system to carry out robust wireless communication under such conditions with increased throughput and capacity, improved signal quality, increased resistance to multipath fading.
Accordingly, it is a primary object of the present invention to provide a method for wireless communications that:
1) is robust and has improved signal quality and increased resistance to multipath fading;
2) is suitable for varying channel conditions from pure Rayleigh to pure Rician with varying delay spread;
3) does not require channel state information;
4) has increased throughput and capacity compared to prior art methods.
These and other objects and advantages will be apparent upon reading the following description and accompanying drawings.
These objects and advantages are attained by a present method for wireless transmission of a digital bit stream. A digital bit stream is converted into a plurality of symbols. The symbols are sent to each of at least two diversity branches. The symbols are multiplied by a phase factor. The multiplication can be performed before or after the symbols are sent to the diversity branches. Finally, the symbols are transmitted from at least one of the diversity branches. Preferably, the symbols are transmitted from all the available diversity branches.
Alternatively, the symbols are grouped into frames. In this case, multiplication is performed by multiplying the frames by a matrix comprising phase factors. Also, the symbols can be cyclically shifted within the frame.
The phase factors can also be randomly selected. Preferably, the symbols are randomly selected when there is no available channel information (i.e. channel characteristic measurements are not available).
The symbols can also be multiplied by an amplitude factor. The amplitude factor can be randomly selected. The amplitude factor can be adjusted according to a channel characteristic (quality) measurement).
Each diversity branch may have a separate antenna.
Preferably, the phase factors are adjusted according to channel quality or channel characteristic measurements.
Another method according to the present invention includes converting a digital bit stream into a plurality of symbols grouped into a frame. The frame is sent to at least two diversity branches. A linear phase shift is incorporated into the frame. Finally, the frame is transmitted from at least one diversity branch. Preferably, the linear phase shift is different for each diversity branch. Also preferably, the frame is transmitted from all the diversity branches available.
Yet another method according to the present invention includes converting a digital bit stream into a plurality of symbols. The symbols are then converted into a frame using a serial to parallel converter. A cyclic shift is performed on the frame and the frame is sent to at least two diversity branches. Finally, the frame is transmitted from at least one diversity branch. Preferably, the cyclic shift is different for each diversity branch.
The frame can also be multiplied by phase factors and amplitude factors. The phase and amplitude factors can be randomly selected and can be adjusted in response to channel characteristic (quality) measurements.
The present invention includes an apparatus for adaptively selecting a wireless data transmission technique. The apparatus has a measuring means for measuring a wireless channel characteristic. Many different characteristics can be measured. The apparatus has a controller for receiving measurements from the measuring means. The apparatus also has a plurality of multipliers for multiplying the symbols by phase and amplitude factors. The multipliers are controlled by the controller. The multipliers are controlled in response to measurements made by the measuring means. The multipliers can be matrix multipliers. The apparatus can have a serial to parallel converter for grouping the symbols into frames.
The measuring means can measure many different channel characteristics or channel quality parameters including K factor, channel coefficient, signal quality, signal to interference noise ratio, level crossing rate, level crossing duration, and antenna correlation factor.
The present invention also includes an apparatus for wireless transmission of a digital bit stream. The apparatus has a means for producing a frame from a group of symbols. Also, the apparatus has at least 2 matrix multipliers for receiving and multiplying the frame by a matrix. The apparatus also has diversity branches for receiving multiplied frames and transmitting the frames. Each diversity branch is in communication with at least one matrix multiplier.